(a) Field of the Invention
The invention relates to video data source systems, particularly to analog back ends in video data source systems.
(b) Description of the Related Art
Since the entering of the 20th century, the development of television technology and its applications has proven that it is now part of human life and core of entertainment. Because of the improvement of display technology in recent years, providing massive data and high-definition images has become the focus of the future development of the television industry. Please refer to FIG. 1, showing a schematic diagram of a typical television system and its video source. As shown in FIG. 1, the video source system 110 transmits video data in analog signal format to a television system 120 for reproducing video images. Although digitized transmission interfaces have already become available, analog transmission interfaces are still most commonly used.
The video source of the television system 120 can take many forms, such as a DVD player, set top box, and even various game consoles. In general, the components of the video source system 110 comprise a video encoder 112 for performing image data encoding, and a digital-to-analog converter (DAC) 114, which converts the encoded digital signal into an image analog signal and outputs the image analog signal.
The television system 120, such as a LCD TV or other flat panel television system or digital television system, which is becoming the main stream applications, receives an image analog signal transmitted from the video source, converts it into digital format with an analog-to-digital converter (ADC) 124, and performs decoding operation with a video decoder 122 for further image processing and displaying.
There are many types of video encoding format. The most commonly available ones are: RGB signal format, CVBS signal format, luminance/chrominance (Y/C) signal format, and color difference (YPrPb) signal format and so forth. Therefore, the analog transmission interface for transmitting video signals between the video source system 110 and the television system 120 can also be one of several types, such as Digital Visual Interface (DVI), which comprises a Display Data Channel (DDC), for transmitting signals in RGB format; AV interface for transmitting signals in the CVBS format; S-video interface for transmitting signals in the Y/C format; and color difference video interface for transmitting signals in the YPrPb format.
As shown in FIG. 2, in the case of the video analog signal Aout of Y channel while the video encoding format is YC signal format, the video analog signal Aout comprises a Y signal and a synchronizing signal (Sync signal). The dynamic range of the common Y signal is 0.7V while that of the Sync signal is 0.3V. Thus, it requires a total dynamic range of 1V.
As shown in FIG. 1, in a commonly seen application, each of the power source resistor RS of the output terminal of the video source system 110 and the load resistor RL of the input terminal of the television system 120 has a resistance of about 75Ω. It is therefore required that the output of the digital-to-analog converter 114 has the capability to drive a resistance of 37.5Ω, which is the resulting resistance of the two parallelly-connected resistors RS and RL, and meanwhile has the dynamic range of 1V. It can be seen in the figure that the digital-to-analog converter 114 has to directly drive the load of 37.5Ω, and in order to drive such a relatively small load, the digital-to-analog converter 114 needs to output a relatively large current, which in turn results in excessive power consumption of the digital-to-analog converter 114.
In the conventional design, a buffer amplifier outside the chip, usually disposed on the printed circuit board, is used to provide the required driving power. However, such design still suffers the problems of increasing costs of external circuitry and increasing power consumption due to the added buffer amplifier.